Serotonin immunoreactive structures of the rat central nervous system (CNS) were detected by the recently developed silver-intensified peroxidase-antiperoxidase complex (SI-PAP) method at both the light and electron microscopic levels. The silver postintensification of the diaminobenzidine (DAB) chromogen increased the sensitivity of the original PAP method, resulting in a very Golgi-like appearance of serotonin-immunopositive neuronal elements. The metallic silver and gold deposited onto DAB-labeled organelles, filling out the whole immunoreactive neuron, assures the easy tracing of thin neuronal processes far from the cell body. At the ultrastructural level, metallic grains were seen over immunolabeled structures only, proving the specificity of the silver method. In neurons of the dorsal raphe nucleus, free ribosomes, endoplasmic reticulum, and granules (80-100 nm in diameter) were labeled. Immunoreactive, e.g., serotoninergic, dendrites were seen to receive afferent terminals. The increased electron density of the intensified immunolabel facilitates the ultrastructural recognition of even weakly labeled profiles, while its metallic components (Ag and Au) provide a base for X-ray analysis of the immunolabeled biological specimen.
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